On wireless high-speed, high-quality data services, it is very difficult in practice to receive pure signals without signal distortion or noise. Adverse influences are attributed to a radio channel environment in a wireless communication system. For a wireless communication system, the radio channel environment varies frequently because of white noise, fading-incurred signal power changes, shadowing and interference from other users and multi-path signals. If the data is received in a mobile terminal, the influence further includes the Doppler Effect that occurs due to the movement and frequent velocity changes of a terminal.
Accordingly, great amounts of time and energy have been expended toward minimizing the influence of distortion or noise involved with signal transmission and reception in a wireless communication system. Common techniques in communication systems with unreliable and time-varying channel conditions are AMCS (Adaptive Modulation & Coding Scheme) and HARQ (hybrid automatic repeat request).
AMCS adjusts a modulation order and a coding rate according to changes in downlink channel condition. The downlink channel quality is usually evaluated by measuring the SNR (Signal-to-Noise Ratio) of a received downlink signal at a UE (User Equipment). The UE feeds back the channel quality information to a BS (Base Station) on an uplink. Then the BS estimates the downlink channel condition based on the channel quality information and determines an appropriate modulation scheme and coding rate for a channel encoder according to the downlink channel condition estimate.
HARQ is a retransmission control technique, which is to correct errors in initially transmitted data packets based on automatic repeat request (ARQ) schemes together with a forward error correction (FEC) technique. Schemes for implementing HARQ include chase combining (CC), full incremental redundancy (FIR), and partial incremental redundancy (PIR).
FIG. 1 is a block diagram of a transmitter/receiver in a typical high-speed wireless data packet communication system. Referring to FIG. 1, the transmitter 100 includes an encoder 110, a channel interleaver 120 and a modulator 130. Upon input of information bits, the encoder 110 is operable to encode the information bits at a predetermined coding rate. If coding rate R (=n/k, n is prime to k.) is set to, for example, ½ or ¾, the encoder 110 outputs n coded bits for the input of k information bits. The burst errors, which are often generated on a fading channel, can be prevented by interleaving. The channel interleaver 120 performs interleaving to distribute coded bits having the same information to overcome the shortcoming of the error control coding, and to minimize data loss caused by burst errors. The modulator 130 modulates the interleaved bits in a predetermined modulation scheme, such as QPSK, 8PSK, 16QAM, and 64QAM. The modulated data is transmitted over the communication channel 190. The communication channel is typically a radio communication channel experiencing unreliable and time-varying channel conditions. Preferably, the transmitter 100 can further include a controller to select the coding rate for the encoder 110, and modulation scheme for modulator 130.
The receiver 101 includes a decoder 160, a deinterleaver 150 and a demodulator 140. The demodulator 140 demodulates the received data into a corresponding bit domain sequence. The deinterleaver 150 performs deinterleaving the bit sequence from the demodulator 140, by applying a determined, pseudo-random or random permutation of the input bit sequences which is applied by the interleaver 120. The decoder 160 then decodes the deinterleaved data to output the information bits.
As stated before, the modulator 140 supports various modulation schemes including QPSK, 8PSK, 16QAM and 64QAM with respect to the interleaved bits. In modulator 140, an interleaved data mapped on a modulation symbol, and the symbol mapping refers to designation of symbol positions in a two-dimensional symbol constellation having an I channel along an X axis and a Q channel along a Y axis. As a modulation order increases, the number of bits in one modulation symbol increases. Bits mapped to one modulation symbol have different transmission reliabilities according to their positions. With regard to transmission reliability, two bits of a modulation symbol representing a macro region defined by left/right and up/down have a relatively high reliability in an I (In Phase)-Q (Quadrature Phase) signal constellation. The other bits representing a micro region within the macro region have a relatively low reliability.
FIG. 2 illustrates an exemplary signal constellation in 16QAM. Referring to FIG. 2, one 16QAM modulation symbol contains 4 bits [a3, a2, a1, a0] in a reliability pattern [H, L, H, L] (H denotes high reliability and L denotes low reliability). That is, the two bits [a1, a3] have a relatively high reliability, and the two bits [a0, a2], a relatively low reliability. FIG. 3 illustrates an exemplary signal constellation in 64QAM. Referring to FIG. 3, one 64QAM modulation symbol contains 6 bits [a5, a4, a3, a2, a1, a0] in a reliability pattern [H, M, L, H, M, L] (M denotes medium reliability).
In conventional HARQ, however, initial transmission bits and their retransmission bits are the same in reliability. Bits mapped to a low reliability position still have the low reliability at retransmission and the same occurs to bits mapped to a high reliability.
In IEEE 802.16 standard, one of the forward error correction (FEC) schemes is duo-binary turbo code called convolutional turbo code (CTC). FIG. 4 illustrates a block diagram of conventional CTC encoder. CTC encoder 400 comprises a CTC interleaver 410, a first constituent encoder 421 and a second constituent encoder 422. Upon input of A code and B code, the CTC encoder 400 outputs six code groups including A code, B code, Y1 code, W1 code, Y2 code and W2 code, wherein A code and B code are systematic parts. Y1 code and W1 code are the parity parts generated by the first convolutional encoder 421. Y2 and W2 are the parity parts generated by the second convolutional encoder 422.
FIG. 5 illustrates a schematic view of conventional channel interleaving scheme for CTC encoder. The channel interleaver performs following operations: bit separation 51, subblock interleaving 52, bit grouping 53 and bit selection 54. In operation of bit separation 51, the encoded bits outputted from CTC encoder 400 are sequentially distributed into six subblocks. A code subblock 551, B code subblock 552, Y1 code subblock 553, Y2 code subblock 554, W1 code subblock 555 and W2 code subblock 556. These six subblocks are respectively inputted to subblock interleaver 591, 592, 593, 594, 595 and 596 in operation of subblock interleaving 52. In bit grouping 53, the interleaved A and B subblock sequences are grouped directly, Y1 and Y2 subblock sequences are bit-by-bit multiplexed, and W1 and W2 subblock sequences are bit-by-bit multiplexed. In bit selection, the grouped bits are selected continuously and circularly to generate subblocks and fed to the modulator.
However, this channel interleaving scheme has some disadvantages. First, some contiguous coded bits are mapped onto the bit location with the same level of reliability on the constellation. Besides, when 16-QAM is considered, subblocks Y1 (W1) and Y2 (W2) are always mapped into more and less reliable bit location respectively, as shown in FIG. 6. Third, the reliability distribution of systematic and parity bits corresponding to the same information bit is not uniform.